The invention relates to a method and an apparatus for producing monocrystals by cooling a melted mass of a source material for the monocrystal. Specifically, the invention relates to a method for producing monocrystals of different materials, for example III-V-materials such as gallium arsenide, of large diameter.
Methods for producing monocrystals of different materials, for example III-V-materials such as gallium arsenide (GaAs) are known. The known methods for producing various III-V-monocrystals are disadvantageous in that the form fit of the crystal nucleus required for establishing the crystal orientation in a small diameter portion of the crucible often causes the crystal nucleus to contact the inner wall of the crucible. As a consequence the surface of the nucleus cannot be sufficiently covered with oxidic melted mass, thereby allowing the group of the V-component to evaporate.
The evaporated material condenses in an uncontrolled manner at colder places of the reaction system and freely crystallises thereat. The surface of the nucleus is enriched with the III-valuant components whereby the growth of the monocrystal is prevented. These free crystallites and the decomposed nucleus, respectively, act as additional crystallisation nuclei causing polycrystalline growth. Moreover, the direct contact of the nucleus with the wall of the crucible can result in a local deformation and destruction of the nucleus owing to forces arising from different expansion co-efficients of the materials. This also occurs whenever the layer of boric oxide is not entirely and completely fitted around the grown monocrystal. The result is an undesired crystal orientation and therefore a total loss of the required product. In order to avoid an uncontrolled orientation of the crystal it is required to completely cover the crystal nucleus and the crystal itself with the oxidic melted mass.
Known methods for reducing the dislocation density include the use of nuclei with few dislocations. However, such nuclei are susceptible to thermally induced stresses which may destroy a nucleus by producing additional dislocations. To make a nucleus more resistant against thermally induced stresses it can be imagined to keep the temperature gradient at the place of nucleation very small. The consequence is however, that the position of the place of nucleation is uncertain because of the existing fluctuations. Furthermore, this increases the process time because one is forced to keep low the super heat of the reaction space for supplying the melting heat of the solid base material. With the known method the production of monocrystals of different III-V-materials such as gallium arsenide with large diameter, for example 100, 150 or 200 mm, is not possible or requires a disproportionately high expense.
It is known from documents EP 0 744 476 A2 and EP 0 671 490 A1 to cover, in a first step proceeding the process of crystal growth, the inner wall of the crucible with a solid layer for preventing the direct contact between the boron nitride crucible and the nucleus. However, there is the risk that this layer chips off when the nucleus is introduced into the crucible, because the layer cannot in generally be safely attached to the crucible.
It is the object of the invention to provide an improved method and apparatus for producing monocrystals of different III-V-materials. It is a further object of the invention to provide a method and an apparatus for producing monocrystals of different III-V-materials such as gallium arsenide having a large diameter and few dislocations with high quality in a simple manner.
In order to achieve the above mentioned objects the invention provides an apparatus for producing a monocrystal by cooling a melted mass of a source material for the monocrystal, the apparatus comprising a crucible for receiving the melted mass, the crucible having a first portion with a first cross-sectional area and a second portion with a second cross-sectional area which is smaller than the first cross-sectional area, the second portion forming a nucleus channel with an inner wall, a nucleus having a first part with a first cross-sectional area and a second part with a second cross-sectional area which is smaller than said first cross-section area, the nucleus being disposed in the nucleus channel of the crucible with the second part being aligned with the second portion of the crucible, and an interspace between the inner wall of the nucleus channel and at least the second part of the nucleus whereby the second part of the nucleus is freely standing within the nucleus channel.
According to a further aspect the invention provides a method for producing a monocrystal by cooling a melted mass of a base material for the monocrystal using the above defined apparatus and comprising the step of filling only the interspace between the inner wall of the nucleus channel and the smaller cross-sectional second part of the nucleus with a liquid oxidic melted mass, for example boric oxide (B2O3).
According to a further aspect the invention provides a method for producing a monocrystal by cooling a melted mass of a base material for said monocrystal comprising using a crystallisation nucleus in a crucible having a nucleus channel, the nucleus being resistant to thermal shocks and having a dislocation density of about 10,000-100,000 dislocations/cm2.
The inventive method, apparatus and nucleus ensure the production of monocrystals of different III-V-materials of large diameter with a high yield. It is therefore possible to produce gallium arsenide monocrystals having a diameter of 2xe2x80x3, 3xe2x80x3, 100 mm, 125 mm, 150 mm, 200 mm and greater in good quality. The manufacturers of semiconductor components are therefore provided with a product which fully meets the high requirements for such semiconductor components. The yield obtained in crystal growth is considerably improved by the inventive method.